Piezo-electric resonator



Reissued July 2, 1929.

, UNITED STATES PATENT OFFICE,

WALTER G. CADY, OF MIDDLETOWN, CONNECTICUT, ASSIGNOB TO RADIO C ORPOBA TION OF AMERICA, A GORPQRATION OF DELAWARE.

PIEZO-ELECTRIG RESONATOR.

Original No. 1,450,246, dated April 3, 1923, Serial No. 354.659, filed January 28, 1920. Application for reissue .filed August 6, 1925.

DlVl s1oN B.

The presen t inventio'ii' relates to electrical systems, and more particularly to electric wave or frequency meters. W I

A chief object of the invention is to provide a new and improved method of, and appi-iratus for, measuring electric wave lengths or frequencies.

\Vith this and other objects in view, the invention consists of the in'iprovcd wave-length or frequency meter hereinafter described, illustrated in the a'cconipanyil'ig drawings, and defined in the appended claims.

In the accompanying drawings, Fig. l is a diagrammatic view of a circuit and apparatus illustrating the principles underlying the present invention; and Figs. 2 to 6, inclusive,

are similar views illustrating several forms of the invention. I

For many years it has been known that quartz, tourmaline and certain other crystals, if compressed or otherwise strained in certain directions, respond electrically, so as to exl'iibit positive and negative electrification in certain regions on their surfaces. It has also been known that the converse effect obtains, that is to say,.lhat such crystals, when electrically stimulated, as by being placed in an electric l'ield, become deformed. This phcno menon has been known as piezo-electricity. 1n the specifications and the claims, the action of the electric forces to cause mechanical deformation \\'i ll. for brevity, be called stimmulation; and the development of the electromotiveforce by the deformation will be called response. A large number of both natural and artificial crystals have been found to possess this piezo-electric property. The general knowledge on the subject has so far developed that it is now possible to predict in advance whether a given crystal will. show the pieZo-electrie effect, and in what manner a plate or rod should be 'cutfrom the crystal in order to exhibit this effectto the greatest posare connected with a. source of high frequency current 4 by conductors 3, as shown in Eig. 1,

becomes set into vibration at variousfre-,

quencies of the alternating current. The amplitude of such forced vibrations is exc'eesmgly minute, except when the frequency of the current approximates a natural frequency of mechanical vibration of the plate. Even at this natural frequency the changes in'the dimensions of the plate resulting from its forced. vibrations are so minute as to escape detection under ordinary conditions. They are nevertheless suflicient to exert a marked reaction upon the electric current. At approximately the critical frequency, the absorption of energy in the plate causes the current in the circuit to pass through a minimum. 1f the frequency of the current be increased, for example, then the current traversing the cir hit, as measured by an annneter 5, in series with the coatings, will be found to pass through a maxin'ium at a frequency slightly below the resonant frequency, followed by a minimum at a frequency. slightly above the resonant frequency. If the frequency be decreased, this process will be reversed. The reactions of the crystal upon the current are such that, with a tube-generating circuit, as commonly employed, it is usually impossible to make the circuit oscillate at exactly the natural frequency of vibration of the crystal when the crystal is connected directly in the circuit, but the frequency at which the abovementioned minimum occurs is so close to the natural frequency that, for most practical purposes, it. may be regarded as the natural frequency of the crystal. The theory will be found explained at greater length in the above-named application and in my paper in the Proceedings of the Institute of Radio 'lflngineers, vol. 10, April. 1922.

\Vhen the plate is made from a piezo-electric crystal of good elastic qualities, such as quartz, when its width is small in comparison with its length, and it is mounted in such manner that its vibrations will be damped as a little a. sharp an connected to a source of electric oscillations possible, the reaction is extremely very pronounced. Such a plate,

ofvariable frequency, will respond when one andonly one irequency-neglecting for the moment'overtones-is being generated. It

" forms a piezo-electric resonator, somewhat analo ous to the acoustical resonators of "undesired modes of vibration. may have varlous shapes, or some other form.

Helm oltz. If the natural frequency of the plate has been determined by comparison .with a standardwave-meter, or otherwise, it

may be, in turn, used as a standard for calibrating radio and other high-frequency circuits, anda number of such plates, of dif-' .ferent lengths, may be used for calibratinga high-frequency circuit over as wide a range as desired. 1 It is not necessary that the device comprise a long, narrow plate, although this has 1ts advantages in permittln greater sharpness of tuning, and in securmg greater free-' dom'from effects that might be produced by The plate than a plate may be used, including the entire native or stal itself. In fact, it within 4 the scope of t, e present invention to use other electro-mechanical vibrators than piezo-electric crystals. The onl essential condition is that the vibratorshal be mounted and connected to an oscillating circuit of variable frequency in such a manner as to react electrically upon such circuit at a particular freuency. In general, the more complicated may form part of any tuned oscillatory cir cuit' in which alternating current of variable frequen is flowing. :The wiresflfi-and 16 connect t e'condenser 14 with the remainder of the circuit,'throu h anammeter- 5. At a critical frequency, t e absorption of'energy in the piezo-electnc resonator causes the current in the-ammeter to pass through a minimum. This decrease in current is the greater, the smaller the capacity of the condenser with respect to. that of theresonator.

The current flowingto, the crystal body does not usually sink to asimpleininimum at resonance, but as the frequency-is increased, it-pa'ss'es through a maximum, followed by a minimum, with a possible secondary maximum and minimum lying in-between. In Fig. 5, the current passes through a minimum at resonance in the ammeter 21, and also in the condenser 10. L This is explainedin my said pa er in the Proceedings of the Institute of Radlo Engineers, ages 99-to 101.

Also this figure in icates one way in whichvibrateland to react percepti circuit.

ig. 2, for ex-'- the piezo-electric resonator may be "made to serve as a standard of frequency or of wave length in radio-telegraphy. It is only necessary to connect several suclrresonators, in succession, in place of that shown, eachtime making note of the readings of the condenser or other apparatus at the critical frequency.

A resonator, when used as a frequency standard,should not be connected in series with the oscillating circuit, since its capacity is so small that it would destroy the tuning.

It is usuall connected in parallel with the tuning con enser. The alternating current traversing the circuit thus ,divides,part passing through the condenser 14, Fig. '2 and the remainder through the crystal 1.. The effect of the crystal upon the tuning is then slight.

Such effect as it has upon the tuning may be further reduced by connecting a very small capacity, of a' few micro-micro-farads, in series with the resonator. Often the response is sufliciently strong, evenwhenone or both of the termlnals of theresonator is entirely disconnected, thecapacities between the-Tresonator terminals'and the circuit being sufiicient to effect aperceptible response. By

means of these capacities, the resonator is enabledtoabsorb enough ener y to make it Ely upon the In Fig. 3,-the piez o-electric resonator 1 forms part of an oscillatory-circuit loosely coupled to a tuned circuit comprising a coil 17 and a condenser 10. In parallel with the latter, is a detector 18 and a telephone'receiver 19, or'somel other indicating device may be properly connected with the circuit.

When the resonator is connected in parallel with the coil 20 in the first circuit and the frequency of the alternating current is varied through the critical value, the sudden reaction is not generall necessary that the circuit of coil 17 should e in exact with that con taining 20.

of the resonator upon the current in 20 pro-. 1 duces an audible click in the telephone 19., It

Fig. 4 illustrates. the same plan :as the preceding figure, but, in this case, the piezoelec-' tric resonator is in parallel with the con-. denser 10 inthe recelvmg circult, instead of being in parallel with coil 20.

The operation is the same as described in connection 'with Fig. 3, except that the crystal reacts upon the receiving circuit, comprlsin'g the coil 17- and the condenser 10;

circuitds used for the standardizing of frequency. The tuning condenser in parallel withthe crystal and the resistance of the circuit should have as low values as possible.

Fig. "5 is similar to Figs. Band 4, except that, instead of the detector and the telephone, there is shown a high-frequency ammeter 21.

, 12c This connection of the resonator inpa'r'allel j with the capacity in a secondary circuit is found, in practice, to .be the best when the By varying the frequency of the current in coil 20, and keeping the secondary circuit, which, comprises the coil 17 and a variable condenser 10, in electrical resonance with the current in 20, it is possible to observe quantitatively the manner in which the current in the ammeterpasses through a minimum at the critical frequency, and to determine the settings of condensers and other instruments a as the before-mentioned click. The pitch and corresponding to this degree of precision.

For many purposes, it is immaterial whether the crystal is in the oscillatory circuit frequency, with a high comprising the coil 20, as shown in Fig. 3, or

in the circuit comprising the coil 17, as in Figs. 4 and 5. When the highest precision issought, however, it is better to mclude the crystal in a different circuit from that which it is desired to standardize. With the crystal connected as in Fig. 3 or 4, the operation oftuning the generating circuit comprising the coil 20, so as to bring it into synchronism with the natural'frequency of the resonator, is asfollows: When the 'frequency of the'current inthe coil passes through the critical value,

the .crystal is set into vibration, andit continues vibrating for a fraction. of a second evenafter the frequency of the impressed alternating current has assumed a different value. 'The crystal, by its vibrations, generates a pie'zo-electro-motive force, and two frequencies are then present in the secondary circuit :one, that of the impressed alternat- -ing current as determined by the electrical constants of the circuit; and the other, that of-the vibrating crystal. If the frequencies of these-two currents are near each. other,- they may differ, say, tothe extent of a few hundred cycles,the currents will combine, accordingto the heterodyne principle,'to produce a beat-note'of musical'quality, which may be detected in the telephone receiver 19 duration of the beat'note depend upon the extent to which the frequency is varied after the resonator hasbeen set into vibration, and I also upon the rate at which the vibrations of -the resonator die away.- If this rate is very rapid, then, on varying the. frequency through the critical value, only a short'click, instead of abeat note of recognizable'pitch, will be heard. This phenomenon is, described in my paper abovermentioned, particularly page When the resonator is used as a frequency standard, connected to a secondary circuit as in Figs. 2, 4 or 5, the indicating device is,'as

a rule, either a current-measuring device in themain circuit, like the ammeter in Figs. 2 and 5, or else a potential-operated device, like the detector and telephone receiver in Fig. 4. In the case of Figs. 2 and 5 the current flow ing throu h theindicating device sinks to=a, simple minimum at the natural frequency of the resonator. These facts are-.cxplained.,

more'at length in in connection with ig. 5 of that paper and on page 101.

my said paper, particularly Fig. 6 represents aform of circuit in common use for generating high-frequency cur- .rents by means of a three-element vacuum tube 22. In this figure 23 and 24 represent, I

and its capacity is so small as not to intro-' duce a perceptible error. I

' Currents of varying frequency may begob- Condenser 10, r V

tained by varying'the capacity of the condenser 10." Whenever the frequency passes. through the natural frequency of vibration of the resonator, the latter is set into vibration; As long as these vibrations continue, there will be two frequencies present in the system; one due to the-electrical tuning, as

determined by the settin of the condenser 10, and the other due to t e vibration of the resonator. The two frequencies will, as before described, cause a beat note to be heard in the receiver 19. In many cases, moreover, it is possible to detect-the resonant frequency with a very loose couplin of the resonator,

as when only one side of the resonator is connected, or even whenboth sides are disconnected, as above descrlbed. In such cases, of

course, the crystal is nevertheless connected with the circuit through the minute ca acities between the resonator terminalsan adjacent portions of the circuit. g

The above-mentioned beat note" is heard both when the capacity-of the condenser '10 is increased, thus decreasing the frequency, and when the said capacity is decreased, thus increasing the frequency. The two settings of the condenser 10 at which the clicks or beats occur'are not exactly the same for they occur at the breaks 3 and 5,- Fig. 6 of Patent No. 1,472,583. Theirmean may betaken as the setting corresponding to the crystal frequency, for they are both exceedingly close to the natural frequency of mechanical vibration ofthe crystal. By using a narrow crystal, or by disconnecting one orboth termlnals, the reaction of the crystal upon the circuit may be lessened. 'The clicks, though thus .made fainter, are nevertheless amply r loud and sharp, while the two condenser set tings may thus be broughtso close together 1; as to be practically indistinguishable. Even U when a largecrystal is employed, so that the settingsof the condenser 10 are made wide apart the clicks still occur close tothe [natthe use of impact excitation.

ural frequency of the crystal. The wider apart the condenser settings are, the better will the crystal act as a stabilizer, as described in the said Patent No. 1,472,583. At the frequency at which the click occurs, or, at least, at a frequency very close to this frequency, the current is at a minimum in the coils 26 and 28 and in the condenser 10. The current to the crystal itself, on the other hand, may be at a minimum or a maximum at, or close to, the click, depending upon whetherthe frequency is decreasing or increasing. The circuit of Fig. 6 constitutes a very good frequency-meter circuit.-

In the description and illustrations given above, I have assumed that a supply of undamped alternating current is available. \Vhile the best results are obtained in this way, it is possible to use the resonator as a standard of wave length also when damped waves only are available, as for example, from a buzzer circuit. The reaction is then much less pronounced, owing to the fact that a damped train of waves contains, not a single frequency, but a combination of many frequencies. If, however, damped waves must be used, it is best to make the decrement as small as possible, for example, by Some of the circuits above described, as for example, the circuit shown in Fig. 4, or proper modifications of them, may then be employed.

Vhile the fundamental frequency of the plate'or rod will usually give the strongest reaction, the various overtone vib 'ations can also be employed, giving reactions at frequencies approximately two, three or more times the fundamental. Or, if the plate has other dimensions comparable with its length, or if some other shape than that of a plate is used, some other mode of vibration, giving a still different frequency, may be utilized. When the vibrating unit has once been suitably prepared and mounted, its resonant frequencies are, to a high degree of precision, fixed for all time.

It will be understood that the invention is not restricted to the exact embodiments thereof that are illustrated and described herein, as other modifications will readily occur to persons skilled in the art, and all such are considered to fall within the spirit and scope of the invention, as defined in the appended claims.

\Vhat I claim as my invention is:

1. The method of indicating the frequency of current flowing in an alternate current circuit which comprises impressing said current upon a frequency standard member having a natural period of mechanical vibration, reducing the current flowing at" a frequency equal to that of the natural periodof the member, andindicating the said reduction in current.

2. A frequency indicating device comprisdicating the electrical reaction.

ing a circuit adapted to be traversed by an alternating current ofunknown frequency, a piezo-cleclric body connected to said circuit, said body being adapted to be stimulated mechanically by an alternating current of a predetermined frequency only and to react electrically upon said circuit when so stimulated. and means to indicate said reaction.

- A frequency indicating device comprising an alternating-current circuit in which currents of different frequencies are adapted to flow, an elcctro-meclianical vibrator connected to said circuit, said vibrator having one or more definite natural frequencies of mechanical vibration, the vibrator being adapted to vibrate mechanically when stimulated electrically and to respond electrically when vibrated mechanically and to react upon the current flowing in the circuit, the vibrator being so designed as to be set into vibration by energy derived from the circuit at one of the said frequencies, and means for indicating the said reaction.

at. A frequency indicating device comprising an alternating-current system, a piezoelectric body so designed that a natural frequency of mechanical vibration of the body shall be substantially equal to a predeter mined frequency of the. current flowing in the system, said body being connected to said system whereby the body will react elec-' tric-ally upon the current at substantially the predetermined frequency, and means for in- 5. A. frequency indicating device comprising an altemating-current circuit, a piezoelectric body so designed that a natural frequency of mechanical vibration ofthe body shall be substantially equal to a predeterinined frequency of the current flowing in the circuit, said body being connected to said circult whereby the body will react electrically upon the current at the predetermined, frequency, means for mounting the body so that its mechanical vibrations shall be damped to substantially the least possible extent, and means for indicatingthe electrical reaction.

6. A frequency indicating device comprising an alternating-current circuit, an electromechanical vibrator loosely coupled to the circuit, the vibrator being adapted to'respond electrically when vibrated mechanically and being so designed as to react electrically upon the current flowing in the circuit at a predetermined frequency of the current, and means for indicating the reaction.

7. A frequency indicating device comprising an alternating-current circuit, an electromechanical vibrator loosely coupled to the circuit, the vibrator being adapted to vibrate mechanically when stimulated electrically and to respond electrically when vibrated mechanically, and being so designed as to react electrically upon the current flowing in the circuit at a. predetermined frequency of nected in the current, and means for indicating the reaction.

8. A frequency indicating device comprising an alternating-current circuit, a piezoelectric body loosely coupled to the circuit, the body being so designed as toreact electrically upon the current flowing in the cir-' cuit at a predetermined frequency of the current, and means for detectin the 'reaction.

9. A frequency indicating evicej comprising an alternating-current system having a reactance, an electro-mechanical vibrator adapted to respond electrically when vibrated mechanically, the vibrator being conarallel with the reactance and so designed t at it shall react electrically upon the current flowing in the system at a predetermined frequency of the current, and means for indicating the electrical reaction.

10. A' frequency indicating device comprising an alternating-current system having a reactance, an electro-mechanical vibrator connected in parallel with the reactance, the vibrator being adapted to vibrate mechanically when stimulatedelectrically from said sys-' tem and to respondelectrically when vibrated mechanically,.and means for gradually varying the reaetance, whereby the setting of the reactance may be determined with a highdegree ofprecision; a

11. A frequency indicating device comprising an alternating-current system having a reactan'ce, a piezo-electric body connected-in parallel with the reactance and so designed that it shall reactelectrically upon the current flowing in thesystem at a predetermined frequency of the current, and'means for,de-'

tecting the electrical reaction of the body upu on the current.

12. A frequency indicating device .compris ing an alternating-current circuit having a 4 variable reactance, whereby the. system may be tuned, an electro-mechanical vibrator forreducing the flow of current in the system, the vibratorbeing adapted to vibrate me chanically when stimulated electrically and adapted to be received, an electromechanical vibrator connected in said circuit adapted to res 0nd electrically when vibrated mechanical y, the -vibrator beingso designed that a natural frequenc of the vibrator all be substantially equal to the frequency of the current flowing in of mechanical vibration the receiving circuit, and means for indicate ing the response of thevibrator.

14; A frequency indicating device comprisin an alterating-current secondarycircuit in I;

w ich currents of variable frequency are adaptedito be received, an electro-mecha'nical vibrator connected in said circuit adapted to be vibrated mechanically when stimulated electrically by said currents and to respond quency of mechanical vibration of the vibrator shall besubstantially equal to the frequency of the current flowing in the receiving circuit, and means for indicating the re-,

sp onse of the vibrator.

. 15. A frequency indicating device comprising an oscillatingcircuit havin a source electrically when vibrated mechanically, the

vibrator being so designed that a natural fre-.

of electric oscillations of variable requency,

apiezo-electric body connected to said source and designed to react electrically upon the 1 circuit at a predetermined frequency of the oscillations of the circuit, and means for indicating the electrical reaction.

16. A frequency indicating device com prising an alternating-current system in which alternating current of predetermined fre uency is adapted to flow and an electromec anical vibrator coupled to, said system and adapted to respond electrically when vibrated mechanically by the said current, the vibrator being so designed that the frequency caused by the said electrical response of the vibrator shall produce beats with thepredetermined frequency.

17. A frequency indicating device comprising an alternating-current system in which alternating current of predetermined freqxiliency is adapted to flow. and an electromec anical vibrator coupled to said system and adapted to vibrate mechanically when stimulated electrically and to produce electrical oscillations when vibrated mechanically, the vibrator being so designed that the frequency caused by the vibrations of the vibrator shall be of slightl difi'erent' frequency than the said pre etermined frequency, whereby beats are produced. therebetween. i

18. -A frequencyfindicating device comprising a primary circuit, a secondary circuit in which the fre uency may be varied,

an electro-mechanica vibrator connected with one of the circuits, the vibrator being adapted to vibrate mechanically when stimulated electrieall quencyofsaid electrical response being substantially equal. to the frequency of the currentin the said one circuit, whereby a reaction is produced upon the current flowing in the second circuit, and means for indicating the reaction. i I 19. A frequency indicatingdevice comand to respond elec-fl 'trically when vibrate mechanically, the fre- 1 prising two coupled circuits in which cur rents of different frequencies are ada ted to flow, a piezo-electric body connected with one of the circuits, said body being adagted to be stimulated by the current flowing in one of said circuits to produce an electrical reaction upon the current flowing in the other of said circuits, and means for indicating the said reaction.

20. A frequency indicating device comprising two circuits, a piezo-electric body connectedwith one of the circuits, thcbody being so designed that a natural frequency alternating current ofthe sal signature. v

WALTER. G. GADY. 

